Heater descaling

ABSTRACT

The descaling of heater tubes is effected by alternately subjecting the deposited scale to oxidation and reduction techniques. Oxidation is preferably effected in a vaporous atmosphere of 1.0 to 15.9 percent oxygen, and reduction in an atmosphere of 5.0 to 50.0 percent hydrogen. The heater is flushed, after either the reduction step, or after both oxidation and reduction, with steam and/or nitrogen to remove loosened particles of scale.

llmted States M30113 11 1 1111 3,732,123

Stolfa et al. 1 May 8, 1973 54 HEATER DESCALING 2,347,527 4 1944 Vanderbilt 134 22 ['75] Inventors: Frank Stolfa, Park Ridge; Laurence gg l zf' O. Stine, Western Springs; James R. 3 5 4/1970 Happe] et Deering, Prospect Heights; Douglas G. Linden, Hoffman Estates, all of II].

Universal Oil Products Company, Des Plaines, lll.

1 11611; 4 Dec. 21, 1970 Appl. No.: 100,507

Assignee:

References Cited UNITED STATES PATENTS 7/1942 Turin ..l34/2 UX Primary Examiner.loseph Scovronek Att0rneyJames R. Hoatson, Jr, and Robert W. Erickson [57] ABSTRACT The descaling of heater tubes is effected by alternately subjecting the deposited scale to oxidation and reduction techniques. Oxidation is preferably effected in a vaporous atmosphere of 1.0 to 15.9 percent oxygen, and reduction in an atmosphere of 5.0 to 50.0 percent hydrogen. The heater is flushed, after either the reduction step, or after both oxidation and reduction, with steam and/or nitrogen to remove loosened particles of scale.

10 Claims, No Drawings HEATER DESCALING APPLICABILITY OF INVENTION Our invention involves a method for removing scale from heater tubes. More particularly, the present invention is directed toward effecting the removal of scale from heater tubes having served as charge heaters for various petroleum refining processes. Although well-suited for use in descaling heaters which have been employed in processes for dehydrogenation, hydrogenation, catalytic reforming, hydrocracking, isomerization, etc., the method is most advantageous when applied to a process for hydrorefining.

Hydrorefining processes are effected for the principal purpose of removing various contaminating influences from a wide variety of hydrocarbonaceous charge stocks. Such charge stocks include naphtha fractions, kerosene fractions, light and heavy gas oils, both vacuum and atmospheric, and extremely heavy material commonly referred to in the art as black oils. The latter are not only contaminated by the inclusion of high molecular weight sulfurous and nitrogenous compounds, but also contain high-boiling (above 1,050 F.) asphaltenes, some of which are complexed with metallic components principally comprising nickel, vanadium and iron. The ever-increasing demand for lower-boiling hydrocarbon products has currently created a need for voluminous conversion of the high-boiling material, especially black oils which are found in abundance.

The prior art 'has long recognized that contaminated feed stocks, regardless of boiling range, must be cleaned-up in a hydrorefining process prior to subsequent conversion into more desirable and valuable products. Accompanying the development of hydrorefining processes, have been methods for both catalyst regeneration and equipment cleaning, the latter primarily with respect to charge heaters. Current methods for descaling charge heaters utilize oxidation, or burning, sometimes followed by acidizing. These methods leave much to be desired, and the time intervals between required shut-downs steadily decrease. This is especially true with respect to a charge heater having experienced black oil service. As hereinafter indicated, the heater tube deposit, or scale, is of a character which does not lend itself to removal by present-day burning techniques, with or without acidizing.

OBJECTS AND EMBODIMENTS A principal object of our invention resides in a method for descaling heaters. A corollary objective is to remove scale from heater tubes having seen service in processes charging hydrocarbonaceous black oils.

Another object of the'present invention is to increase the time interval between shut-downs for equipment maintenance.

These, as well as other objects, are accomplished through the use of our invention, one embodiment of which involves a method for descaling a heater which comprises a series of alternating scale oxidation and reduction techniques effected at a temperature in the range of from 950F. to about 1,250F.

In another embodiment, more than one series of altemating oxidation and reduction techniques are conducted, the first being at a temperature in the range of Oxides, sulfides and sulfates of other metals.

about 950F. to about 1,l50F. and the succeeding oxidation and reduction techniques at a temperature from about l,050F. to about l,250F.

Other objects and embodiments will become evident, to those having the requisite expertise in the art of petroleum refining techniques, from the following detailed description of our invention.

SUMMARY OF INVENTION Present-day techniques for removing scale from the interior walls of heater tubes, utilizing oxidation, with or without acidizing, have varying degrees of success depending upon the process in which the heater has seen service. In general, with the heavier, more contaminated charge stocks, scale will necessarily be removed with greater frequency. With respect to black oils, scale removal must be undertaken with the greatest frequency. This is due not only to the extreme degree of contamination existing in black oils, but also to the difference in character of the scale which results. This has been learned through experience in a commercially-scaled unit designed for the conversion of a vacuum residuum having a gravity of 8.8 API, and an ASTM 20.0 percent volumetric distillation temperature of 1,055F. The unit was designed to process about 10,000 Bbl./day of charge stock, other properties of which are presented in the following Table I:

TABLE I Vacuum Residuum Properties Gravity, "APl 8.8 Distillation, D1 160, "F.

Initial Boiling Point 690 2.0% 860 5.0% 950 10.0% 1000 20.0% l 05 5 Sulfur, wt.% 3.0 Nitrogen, total ppm. 4300 lnsolubles, wt.% 6.5 Total Metals, ppm. l00

The heater tube scale, after three burnings over an approximate 13-month operating period, was actually affixed to the interior of the tube walls in two distinct layers. These were 1) an extremely hard black inner layer bonded to the metal, and (2) a softer brown'outer layer bonded to the inner layer. The analytical analyses of the two. layers is presented in the following Table II:

TABLE II Scale Analyses Component, wt.% lnner Outer Carbon 42 9 Hydrogen l I Ferrous Sulfide 36 Pyrites 9 39 lron Oxide 29 Miscellaneous 12 23 Notwithstanding the above-mentioned three burnings, this scale approximated three thirty-seconds inches, with the outer layer being about one thirtysecond inches. To those cognizant of the principles and techniques involved with tube heaters, it will be readily ascertained that such a condition presents insurmountable difficulty in maintaining the tube skin temperatures at a reasonably safe level i.e., generally a maximum of about 1,150 to 1,250F. Analyses further indicated that the thermal conductivity of the scale was about 0.9 BTU/(Hr.) (Ft?) (F./in.) whereas that of the material of construction (No. 347 8.8.) was about 1 10. Recorded tube skin temperatures reached 1,3 50F. as a result.

Our invention, for effecting the removal of this scale involves not merely an oxidation technique, but a series of alternating oxidation and reduction techniques. The use of the term series," in the present specification and appended claims, is intended to connote one or more oxidation techniques, each of which is followed by a reduction technique. Thus, for present purposes, a single oxidation followed by a single reduction is considered a series.

Oxidation and reduction are effected at a temperature in the range of 950F. to about 1,250F., and preferably from 1,050F. to about 1,250F. When more than one series is conducted, the first may be effected at a lower temperature i.e., 1,000F. while the succeeding oxidations and reductions are carried out at a higher temperature i.e., 1,100F.

Oxidation is effected under an imposed pressure in the range of about psig. to about 100 psig. in a vaporous atmosphere of 1.0 percent to about 15.0 percent oxygen, on a mo] basis. It is understood that this range is applicable when the greater share of burning is being effected, as evidenced by the quantity of carbon dioxide in the effluent gases. 'As the carbon dioxide content attains a value of nil," the concentration of oxygen may be increased until a level of 100.0 percent is attained. This high oxygen content is not essential to our invention, but may be employed to further insure complete scale removal. Caution must be exercised in not increasing oxygen content while carbon dioxide emanates in the effluent gases. The concentration of oxygen may be readily controlled through the use of steam or nitrogen in admixture therewith, steam being preferred.

Following the oxidation technique, the heater tubes are flushed with either steam or nitrogen, steam being preferred for this purpose. When no loosened scale appears at the outlet of the heater, the tubes may be dried with air and subsequently purged with nitrogen prior to conducting the reduction technique. Reduction is effected in the pressure range above set forth, and in a vaporous atmosphere of 5.0 to 50.0 percent for a period of from minutes to 10 hours, and is followed by flushing, preferably with steam, to remove loosened scale. The concentration of hydrogen in the reduction step, and the duration thereof is generally determined by the scale formation and its removal rate. A relatively short time and low hydrogen concentration, initially, is recommended since too much scale removal at one time may result in plugging of the tubes.

Analyses have shown the composition of the two layers constituting the scale to be different. Notwithstanding this difference, the method of the present invention proves successful as hereinafter illustrated. The difference in chemical composition of the metalbound black layer and the brown outer layer, which is in contact with the oil being processed, is difficult to explain. The black inner layer contains approximately 4.5 times as much carbon as the outer layer; the carbon from the latter appears to be more easily removed. lron compounds in the inner layer are almost all sulfides, with some pyrites, but little iron oxide. The outer layer is a mixture of pyrites and iron oxide, with little, if any, ferrous sulfide. One possible explanation of the peculiar composition of the two layers resides in the prior burning history of the charge heater from which the scale had been taken. As previously stated, this particular heater had been burned by prior art methods three times within an approximate 13-month operating period. If each time a porous layer of iron oxide had been left on the tube wall, the succeeding start-up would fill the interstitial spaces with oil which would coke-up immediately. Continued operation would then result in deposition of another layer of scale of the composition of the outer brown layer until the next burn operation. This would account for the fact that the inner layer was virtually twice as thick as the outer brown layer.

Although not essential to our invention, since the descaled tubes are virtually clean to the original metal wall, it is considered good practice to acidize the tubes as commonly performed in the prior art. However, in accordance with our invention, the acidizing is effected using a 1.0 percent to about 5.0 percent citric acid solution which has been buffered to a pH from about 2.0 to about 6.0.

While hydrogen constitutes the preferred reducing agent, either producer gas, or the off-gas from a catalytic cracking unit may be utilized. Both of these comprise up to about 15.0 percent carbon monoxide, another vaporous reducing agent. These could be employed where hydrogen is either scarce, or not at all available.

ILLUSTRATIVE EXAMPLE A section of the scaled heater tube above described was removed, and four small sections about 2 /2 1 /2 inches were obtained therefrom. These small sections were obtained from a larger center section which had been partially oxidized at about 1,000F. during a test to determine the accuracy of the thermocouple which had been indicating the excessively high skin temperatures. The thermocouple proved to be accurate to within 7F. One of the smaller sections was set aside as a blank, and a second section subjected to 10.0 percent sulfuric acid at 175F. for a period of about four hours. Scale solubility was only 3.16 percent by weight.

The third and fourth sections were used in effecting the oxidation-reduction techniques of the present invention. The equipment consisted of a furnace with a variac control and a quartz tube with inlet and outlet manifolds. A pyrometer was used to calibrate the control dial. The two sections were centered in the tube within the furnace which was heated to a temperature of 1,000F'. Air was passed through the tube and over the two sections at a rate of cc./min. for about 18 hours. The tube was then purged with nitrogen for a 1- hour period. Visual inspection indicated that both layers appeared to be a reddish-brown. Hydrogen was then introduced into the tube for three hours, at 1,000F., at a rate of about 300 cc./min. The scale immediately turned black. One section was removed and stored under a nitrogen blanket. The scale on both pieces appeared to be loose on most of the surface.

However, that part of the original black layer close to the metal tube wall was very rigid.

The second piece was subjected to a second oxidation-reduction cycle as above described, but at a furnace temperature of 1,100F. and a hydrogen reduction period of 8 hours. Upon inspection, the scale was found to be extremely loose and would reduce to powder at the slightest touch. The entire layer of scale was homogeneous, in consistency, and there was no evidence of rigid scale near the metal tube wall.

Solubility tests were conducted on the piece which had been subjected to two cycles of oxidation and reduction. The scale was 95.0 percent soluble in 10.0 percent sulfuric acid and 94.0 percent soluble in 3.0 citric acid, at 175F. for 4 hours.

The effect of a 20.0 percent hydrogen atmosphere was determined. Another 2% X 1% inches piece was removed from the heater tube section and placed in the furnace at a temperature of l,l00F. Oxidation was effected using air for 24 hours at 88 cc./min. After a nitrogen purge, reduction was effected using a mixture of 80.0 percent nitrogen and 20.0 percent hydrogen for four hours at 475 cc./min. Solubility tests indicated the scale to be 97.0 percent soluble in 3.0 percent citric acid solution.

The foregoing indicates the method by which our invention is effected, and clearly illustrates the benefits afforded through the utilization thereof in the descaling of heaters.

We claim as our invention:

1. A method for the treatment of a heater employed in the hydrorefining of hydrocarbon oil contaminated with high molecular weight sulfurous and nitrogeneous compounds and high-boiling asphaltenes, which comprises descaling said heater by a series of alternating scale oxidation and reduction techniques effected at a temperature in the range of from 950F. to about l,250F.

2. The method of claim 1 further characterized in that said series of alternating oxidation and reduction techniques is effected at a temperature from 1,050F. to about 1,250F.

3. The method of claim 1 further characterized in that said series of alternating oxidation and reduction techniques is effected at least twice.

4. The method of claim 3 further characterized in that the first alternating oxidation and reduction technique is effected at a temperature in the range of about 950F. to about 1,150F., and the succeeding oxidation and reduction techniques are effected at a temperature from about l,O50F. to about 1,250F.

5. The method of claim 1 further characterized in that said heater is flushed with steam or nitrogen after each of said reduction techniques.

6. The method of claim 1 further characterized in that said heater is flushed with steam or nitrogen after each of said oxidation techniques.

7. The method of claim 1 further characterized in that said oxidation technique is effected in a vaporous atmosphere of 1.0 percent to about 15.0 percent oxygen on a mol basis.

8. The method of claim 1 further characterized in that said reduction technique is effected in a vaporous atmosphere of 5.0 percent to about 50.0 percent hydro en on a mol basis. I

9. 'Fhe method of claim 1 further characterized in that said heater is acidized, after the last of said reduction techniques, with a 1.0 percent to 5.0 percent citric acid solution.

10. The method of claim 9 further characterized in that said solution is buffered to a pH in the range of about 2.0 to 6.0. 

2. The method of claim 1 further characterized in that said series of alternating oxidation and reduction techniques is effected at a temperature from 1,050*F. to about 1,250*F.
 3. The method of claim 1 further characterized in that said series of alternating oxidation and reduction techniques is effected at least twice.
 4. The method of claim 3 further characterized in that the first alternating oxidation and reduction technique is effected at a temperature in the range of about 950*F. to about 1,150*F., and the succeeding oxidation and reduction techniques are effected at a temperature from about 1,050*F. to about 1,250*F.
 5. The method of claim 1 further characterized in that said heater is flushed with steam or nitrogen after each of said reduction techniques.
 6. The method of claim 1 further characterized in that said heater is flushed with steam or nitrogen after each of said oxidation techniques.
 7. The method of claim 1 further characterized in that said oxidation technique is effected in a vaporous atmosphere of 1.0 percent to about 15.0 percent oxygen on a mol basis.
 8. The method of claim 1 further characterized in that said reduction technique is effected in a vaporous atmosphere of 5.0 percent to about 50.0 percent hydrogen on a mol basis.
 9. The method of claim 1 further characterized in that said heater is acidized, after the last of said reduction techniques, with a 1.0 percent to 5.0 percent citric acid solution.
 10. The method of claim 9 further characterized in that said solution is buffered to a pH in the range of about 2.0 to 6.0. 